8.1, 8.2 Confidence Intervals
Problem 1
A report in a research journal states that the average decrease in systolic blood pressure of people on a certain blood pressure medication is 24 mmHg with a margin of error of ±4 mmHg with confidence level C = 95%.
 According to this information, the mean decrease in systolic blood pressure of people on this medication, µ, could be as low as _mmHg.
 If the study is repeated, how large should the sample size be so that the margin of error would be less than 2 mmHg? (Assume σ = 9 mmHg.)
Correct Answers
 20
 78
Problem 2
An online used car company sells preowned cars. For 30 randomly selected transactions, the mean price is 2000 dollars.
 Assuming the population standard deviation for transaction prices is 300 dollars, obtain a 99% confidence interval for the mean price of all transactions. Please carry at least three decimal places in intermediate steps. Give your final answer to the nearest two decimal places. Confidence interval: ( , )
 Which of the following is a correct interpretation for your answer in part (a)? Select ALL the correct answers, there may be more than one.

 99% of the cars they sell have a price that lies inside this interval.
 There is a 99% chance that the mean price of all transactions lies in the interval.
 If we repeat the study many times, approximately 99% of the calculated confidence intervals will contain the mean price of all transactions.
 We can be 99% confident that the mean price for this sample of 30 transactions lies in the interval.
 We can be 99% confident that all of the cars they sell have a price inside this interval.
 None of the above.
Correct Answers
 1858.91, 2141.09
 Choice 2, choice 3
Problem 3
How much money do winners go home with from the television quiz show Jeopardy ? To determine an answer, a random sample of winners was drawn and the amount of money each won was recorded and listed below. Estimate with 94% confidence the mean winning’s for all the show’s players.
43460, 38213, 42876, 38921, 36709, 26735, 35472, 31319, 42833, 27159, 32799, 39271, 44016, 28191, 37087
high end of confidence interval =
low end of confidence interval =
Correct Answers
 High end: 39480.1636458906
 Low end: 33194.6363541094